Vibration is often a problem in rotating machinery wherein a shaft rotates in a set of bearings. Vibration damping devices including an oil film in an annular space between the outer race of a bearing and the housing have been utilized. When the shaft moves off the bearing axis, the oil film exerts a damping force on the shaft. These damping devices are known as "squeeze film dampers." In practice, squeeze film dampers are damping devices used in gas turbine engines to dampen the whirling vibration of rotors. The ability of squeeze film dampers to attenuate the amplitude of engine vibrations and to decrease the magnitude of the force transmitted to the engine frame makes them an attractive rotor support. Also, the energy removed in the squeeze film dampers enhances the stability of the rotor-bearing system.
Whirl vibration results from a number of different operational phenomenon. For example, synchronous whirl is caused essentially by centrifugal forces acting on a mass-unbalanced shaft. The shaft is generally mass-unbalanced because the geometric and inertial axes of the shaft are not identical due to machining tolerances and material imperfections, wear and tear, and in some instances due to residues on the shaft from the working fluid in a turbomachine. When the synchronous whirl frequency coincides with a natural frequency in the rotor-bearing system, the system's vibration amplitudes can become excessive. The system is then said to be in resonance. The system's natural resonant frequency is generally referred to as its critical speed.
Another operational phenomenon is a self-excited whirl. This occurs when whirl vibration is caused by elements within the rotor-bearing system and may cause the system to become unstable. Such self-induced vibration rapidly increases in amplitude and frequently is catastrophic to the bearing system.
Other causes of shaft instability include aerodynamic induced excitations which may originate from pressure variations around the circumference of impellers and seals and from material hysteresis, rubbing between rotating and stationary parts and other such activity common to rotating equipment.
Another undesirable phenomenon in rotor-bearing systems is the jump resonance phenomenon. The rotor, under certain conditions, will exhibit a jump from a certain whirling orbit to another. Such jump phenomenon has been referred to as "bistable" operation of a rotor incorporating a squeeze film damper and indicates the non-linear behavior of squeeze film dampers.
The damping or fluid inertia forces in squeeze film dampers are predicted by Reynolds number. Traditionally, squeeze film dampers were operating at very low squeeze Reynolds number. As the speed of aircraft engines increased and as the trend of using lighter viscosity oils increased, the values of Reynolds numbers for squeeze film dampers, in practice, has increased dramatically. Thus, the effect of fluid inertia became more pronounced.
For low Reynolds numbers, the pressure in the damper is determined by the solution of the Reynolds equation. Squeeze film dampers generally can be classified into two broad categories: short dampers and long dampers. Short dampers are those dampers for which the short bearing approximation to the Reynolds equation applies. The short bearing approximation is justified if the damper is relatively short in the axial direction such that the flow in the damper is substantially axial rather than substantially circumferential. Accordingly, for short dampers the pressure gradient in the axial direction is larger than the pressure gradient in the circumferential direction. The aircraft industry has developed a number of effective short damper designs. Long dampers are those dampers for which the long bearing approximation to the Reynolds equation applies. The long bearing approximation is justified if the damper is relatively long in the axial direction such that the flow in the damper is substantially circumferential rather than substantially axial. Therefore, for long dampers, the pressure gradient in the circumferential direction is larger than in the axial direction. For tightly sealed dampers, the flow of the working fluid in the damper is circumferential rather than axial even if the damper is physically short. In such case, the damper behaves as a long damper.
In practice, short dampers are effective at attenuating the magnitude of the vibratory force transmitted to the engine frame at the operating speed. Long dampers are effective at attenuating the amplitude of vibration of the engine at the critical speed. Heretofore, long damper characteristics and short damper characteristics have not been present at the same time in a squeeze film damper. Moreover, no long damper has ever met with commercial success, because no one has been able to design an effective sealing means for long dampers.
It would be advantageous to have a device for dampening vibration which incorporates the benefits of short dampers and long dampers.
Long dampers have less tendency to exhibit the jump resonance phenomenon, which phenomenon can seriously damage an engine. Long dampers are also capable of sustaining high loads which allows rotors to operate for relatively short periods of time with extremely high levels of unbalance such as with a blade loss during operation. Further, long dampers have better stability characteristics then short dampers, i.e., are more capable of removing energy from the vibrations, and thus tend to stabilize the rotor.
However, long dampers transmit larger forces to the support and are susceptible to the effects of fluid inertia, which can introduce additional critical speeds in the frequency range of interest.
On the other hand, short dampers transmit a smaller force to the support, and, thus, are better vibration isolators and are less susceptible to the effects of fluid inertia.
Thus, if a squeeze film damper could be designed to operate as a short damper when it is desired to have a relatively small force transmitted to the support and to operate as a long damper when a small amplitude of vibration is desired, the squeeze film damper would be more efficient, e.g., rotors could operate safely at higher speeds. However, the prior art, considered as a whole, neither teaches nor suggests how this desirable end could be achieved.